One of the practical examples of light-emitting devices used in displays or other devices is an organic light-emitting diode (OLED) using an organic substance in its light-emitter layer. Use of the OLED enables display devices to be produced by an ink-jet method or other simple processes, to be large yet flexible, and to be higher in brightness and lower in power consumption than liquid crystal displays.
An example of the switching device for controlling the OLED in the display device is an organic field effect transistor (OFET) with an organic substance used in its channel layer. However, producing a display device having the OFET combined with the OLED requires a complicated production process.
Accordingly, recent studies have focused on the light-emitting field effect transistor (LEFET), a device that functions as both the OLED and the OFET. The LEFET, which emits light by itself, allows the on/off state of the emission to be controlled by turning the gate voltage on and off. Use of such a device in display devices makes it possible to generate and control a ray of light with a single device, thereby eliminating the necessity of combining two devices, i.e. the OLED and the OFET. Thus, it is possible to adopt a simpler process to produce a display device having a simpler structure at a lower production cost. Furthermore, the device can be arranged with a higher density so as to improve the resolution of images.
Non-Patent Documents 1 and 2 each disclose an example of the LEFET. The LEFET disclosed in the aforementioned documents is illustrated in the sectional views in FIG. 1. A gate electrode 11 is covered with an insulating film 12, on which a first source/drain electrode 14 and a second source/drain electrode 15 are provided. The first source/drain electrode 14 and the second source/drain electrode 15 are made of gold in both the Non-Patent Documents 1 and 2. The LEFET in Non-Patent Document 2 has adhesive layers 131 and 132 made of chromium, each of which is inserted between the insulating film 12 and each of the first source/drain electrode 14 and the second source/drain electrode 15 in order to adhere these electrodes onto the insulating film 12. A light-emitter layer 16 made of an organic substance is provided on the insulating film 12, where the layer is in contact with the first source/drain electrode 14 and the second source/drain electrode 15. The light-emitter layer 16 of the LEFET is made of (C6H5C2H4NH3)2PbI4, a PbI-based lamellar perovskite compound in Non-Patent Document 1, and is made of tetracene in Non-Patent Document 2.
[Non-Patent Document 1] Masayuki YAHIRO, et al. “Soujou Perobusukaito Jiko-soshikika-maku Wo Mochiita Yuuki FET No Hakkou Oyobi Denki-tokusei (Electrical and optical characterization of organic filed effect transistor using self-organized layered perovskite thin film)”, Technical Report of IEICE, The Institute of Electronics, Information and Communication Engineers, OME2002-54, pp. 37-41 (2002)
[Non-Patent Document 2] A. Hepp et al., “Light-Emitting Field-Effect Transistor Based on a Tetracene Thin Film”, Physical Review Letters, The American Physical Society, vol. 91, No. 15, pp. 157406-1-157406-4, Oct. 10, 2003
This LEFET operates as follows:
The first case assumes that a negative gate voltage VG is applied to the gate electrode 11, as shown in FIG. 1(a). A source-drain voltage VSD is applied between the first source/drain electrode 14 and the second source/drain electrode 15, with the second source/drain electrode 15 being positive. The source-drain voltage VSD is within the range from several tens of volts to one hundred and several tens of volts; this range is higher than voltages applied to normal FETs. Applying the voltage VSD causes the second source/drain electrode 15 to inject positive holes into the light-emitter layer 16. Then, these positive holes are transported toward the first source/drain voltage 14, being pulled onto the insulating film 12 due to the gate voltage VG. Thus, in the present case, the second source/drain electrode 15 serves as the source electrode and the first source/drain electrode 14 serves as the drain electrode. Meanwhile, the first source/drain electrode 14 injects electrons into the light-emitter layer 16. The number of the electrons hereby injected is smaller than that of the positive holes injected from the second source/drain electrode 15. To secure a sufficient amount of electrons, the voltage VSD is set at a high level, as explained earlier. The positive holes and the electrons thus injected recombine within the light-emitter layer 16 in the vicinity of the first source/drain electrode 14. Thus, the light emitter generates light. Turning the gate voltage VG on/off leads to an increase/decrease in the concentration of the positive holes in the vicinity of the insulating film 12. Thus, it is possible to control the on/off state of the recombination of the positive holes and the electrons, or the on/off state of the emission.
The second case assumes that a positive voltage VG is applied to the gate electrode 11, as shown in FIG. 1(b). As in the previous case, the source-drain voltage VSD within the range from several tens of volts to one hundred and several tens of volts is applied, with the second source/drain electrode 15 being positive. This setting makes electrons to be injected from the first source/drain electrode 14 into the light-emitter layer 16. The electrons are then transported toward the second source/drain electrode 15, being pulled onto the insulating film 12 due to the gate voltage VG. Thus, in the present case, the first source/drain electrode 14 serves as the source electrode and the second source/drain electrode 15 serves as the drain electrode. Meanwhile, the second source/drain electrode 15 injects a small number of positive holes into the light-emitter layer 16. The positive holes and the electrons recombine with each other within the light-emitter layer 16 in the vicinity of the second source/drain electrode 15. Thus, the light emitter generates light.
If the light-emitter layer 16 is made of a material having a high level of transport capacity for positive holes, the gate voltage VG should be preferably positive, as in FIG. 1(a), and if it is made of a material having a high level of transport capacity for electrons, the gate voltage VG should be e preferably negative, as in FIG. 1(b).